Topic: Piston circlip won’t fit

[Toby84]

Hi,

Started installing new pistons on my CB750 K7 yesterday. I was surprised when I couldn’t get the circlip in place on number 4. Took it out to check, and it looks like the groove is to far in (maybe about 1mm or so), as I can just barely see it with the pin in place. I tried with pins from the other pistons, and they are all the same lengthwise, so it must be the groove that for some reason is out of place.

I got the new set of pistons when I bought the bike, so can’t really turn to any specific vendor (aftermarket no name). And the cylinder is bored to this set of pistons, 65mm/836cc.

What are my options? First thing I come to think about is to put the pin in a lathe and shave of 1-2mm to be able to put the circlip in the groove. But will this effect balance between the cylinders by removing a few grams of weight?

And just to mention, the other pistons mounted perfectly.

Thanks

[Bodi]

Not sure if balance is that critical. Those pins are rather hard metal and grinding might be required... carbide tooling may cut it ok but be super careful holding the pin to avoid any surface damage - a toolpost grinder is probably your best bet. If you're concerned about the balance, shorten all four to match. A bit of axial play won't be a problem IMO.

[madmtnmotors]

What about Teflon or aluminum buttons in place of clips? Grinding hardened steel can affect the tempered hardness if overheated can it not? If you cut/grind anything off the end of the pin can't you just cut enough at the outer edge of the circumference just to clear the clip leaving a small shoulder on the end of the pin? This would reduce the potential for imbalance by keeping the weight reduction to an absolute minimum. You might have to put that clip in first since the shoulder may "lock" the clip in place once the pin is in position.

[Toby84]

Thanks for great replies.

What about Teflon or aluminum buttons in place of clips? Grinding hardened steel can affect the tempered hardness if overheated can it not? If you cut/grind anything off the end of the pin can't you just cut enough at the outer edge of the circumference just to clear the clip leaving a small shoulder on the end of the pin? This would reduce the potential for imbalance by keeping the weight reduction to an absolute minimum. You might have to put that clip in first since the shoulder may "lock" the clip in place once the pin is in position.

I don't know much about teflon or aluminium buttons, are they commonly used in these kind of engines? Would they be possible to buy off the shelf, or do they have to be custom made? Won't the teflon wear out?

And regarding possible imbalance, would the imbalance likely occur between all cylinders, or would it be enough to loose some weight on number 1 only in addition to 4?

Thanks,
Toby

[madmtnmotors]

I suggest consulting a local engine building machinist. He may have other solutions not covered here yet and will likely be familiar with those solutions that have ben mentioned.

[69cb750]

Your piston is defective, buy a new piston.
Modifying good parts to compensate for bad parts ...

[PeWe]

My old RC 836 pistons had teflon buttons.
Not to be dropped just before cylinderblock will go on.

Better to use good pistons.
If stock, maybe time for tighter new pistons in a fresh bore?
+ 0.5mm
https://www.cruzinimage.net/20...8-0-5mm-over-size-pistons-set/

+ 1.0mm.
https://www.cruzinimage.net/20...over-size-pistons-set-62-00mm/

My K2 has the +0.5mm K7 pistons, K7 cam. Yamiya no numbers 4-4, stock K6 carbs, stock K6 refurbished head.
+56 whp on dyno.

[Maltboy]

Hey Toby, did you measure the distance between the two clip grooves to see if it's shorter than the other pistons? 

[Toby84]

Yes, I’ll definetely call the machine shop who did the new bore on Monday.

A new piston would be a solution, but at the moment I can’t find pistons from this set being sold separately. As mentioned it’s a 65mm piston. I suppose it isn’t just plug and play with a new piston, or am I wrong? Would it work to just find a new 65mm piston?

I measured the four pistons distance from side to groove. 1-3 is pretty consistent 3,4mm. No 4 has 3,4 mm on left side and 5,0mm on right side, so it’s quite a big difference.

[PeWe]

Photo of the piston will help. Chance that someone has an extra piston.
Photo of crown, sides and inside to identify ev markings.

[Toby84]

Photo of the piston will help. Chance that someone has an extra piston.
Photo of crown, sides and inside to identify ev markings.

Thanks for all answers. Everything counts 

PeWe, nice to see another Swede around here.

Here are some pictures. Seems like it could be this kit possibly:
https://www.cruzinimage.net/2017/08/25/69-76-honda-cb750k-836cc-65mm-bigbore-pistons-kit-w-head-gasket/

On the last two you can see the difference in placement of the grooves.

[Toby84]

See the difference..

[69cb750]

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=cb750+65+mm+piston&_sacat=0

[PeWe]

There are some guys from Sweden here
You can get the clip to fit, right?

Those look like Cruzinimage pistons. No hot cam with those.
There is a long forum thread about them.

I usually fit the clip on one side before assembly. Push the wrist pin from easiest side to fit the last clip.

Fit the clip you have doubts about and test with wristpin, push it and see that clip will sit.

If wristpin has a  tight fit heat the piston with a heatgun, oil the holes.

Honda stock clips do not have the G shape, more like an O and look like to sit better.
At my last piston replacement I used the Cruzinimage clips.

[rotortiller]

If you are sure it is the cir-clip groove cut wrong and not the pin I would carefully grind the sucker down to fit properly and  very slightly bevel the end  (remove the sharp edge after grinding to length). That little bit of weight removal is minor in a motor like this. If the cylinder is bored to fit a piston than another new piston may not fit as well. Don't over heat the pin while grinding, maybe hold it in a wet rag during the process. I have seen  much worse done to these engines LOL.

[robvangulik]

Isn't there room enough to cut a new groove in the right place?

[Toby84]

There are some guys from Sweden here
You can get the clip to fit, right?

Those look like Cruzinimage pistons. No hot cam with those.
There is a long forum thread about them.

I usually fit the clip on one side before assembly. Push the wrist pin from easiest side to fit the last clip.

Fit the clip you have doubts about and test with wristpin, push it and see that clip will sit.

If wristpin has a  tight fit heat the piston with a heatgun, oil the holes.

Honda stock clips do not have the G shape, more like an O and look like to sit better.
At my last piston replacement I used the Cruzinimage clips.

In the pictures I have both circlips in, but without the pin in. Might be confusing without that information.

With the pin in, circlip no2 will not sit correctly, and if I push the pin from the other side it will snap out of place.

I’ve tried and compared the different pins and they are all exactly the same. As you can see in the pictures it’s even visible to the eye that one of the grooves in placed different in comparison to that side cut half hole that is under one of the circlips.

Actually I would say yes there is probably room to cut a new groove outside of the other one. But would that be easier or better in any way? Someone said earlier that sideways movement of the pin is not a big problem, right?

[robvangulik]

Actually I would say yes there is probably room to cut a new groove outside of the other one. But would that be easier or better in any way? Someone said earlier that sideways movement of the pin is not a big problem, right?

It would keep the weight the same, and you wouldn't need to do anything to the pin.

[PeWe]

Order an extra set is probably safer.
https://www.cruzinimage.net/2017/08/25/69-76-honda-cb750k-836cc-65mm-bigbore-pistons-kit-w-head-gasket/

I have always got refund on shipping when order since the order use a preliminary cost.
Customs cost and VAT to a minimum when the sender has made sure for lowest cost.